Is RF Energy Still A Source Of Heat?

A line of silicon LDMOS transistors has been developed specifically for applications in RF heating at 2.4 to 2.5 GHz.

Energy from RF and microwave sources has long been used for communications, radar systems, and even surveillance. Most solid-state transistor suppliers have sought to increase the power densities of their devices to serve transmitters in these applications. Given some of the products unveiled at the recent IEEE International Microwave Symposium (IMS) 2013 exhibition in Seattle, WA, however, at least one firm is interested in turning RF/microwave energy into a heat source.

With its display at the 2013 IMS, NXP Semiconductors showed that it is at least as interested in using RF/microwave power devices for heat as for radar and communications (Fig. 1). The firm’s exhibition booths included a number of applications in the 2.45-GHz Industrial-Scientific-Medical band, which focused on heating effects. One of these was an RF spark plug, an alternative to conventional ignition systems for firing automotive engines. Another was an RF plasma lamp capable of delivering portable daylight by means of its 140 lumens/W brightness. Yet another RF/microwave application was a solid-state cooker—an alternative to the traditional tube-powered microwave oven—that can heat food safely, uniformly, and efficiently.

1. This plasma torch is generated using solid-state (LDMOS) devices as the energy source.

Vacuum tubes like magnetrons have long served as the RF/microwave energy sources in microwave ovens and for many industrial heating applications. The non-ionizing radiation passes through the food. But the energy of the RF/microwave emissions is absorbed by the water in the food, resulting in dielectric heating. The vacuum tubes are capable of producing the large amounts of power needed to achieve dielectric heating. Yet they are large and require high-voltage power supplies. In contrast, the possibility of achieving dielectric heating by means of solid-state devices—such as power transistors, with their relatively much simpler power supplies—may even clear the way for portable, battery-powered heating devices based on power transistors.

The ISM-band devices from NXP attracted attention at IMS because they came with not just the one rogue transistor, but a portfolio of devices optimally matched to the 2.45-GHz frequency band. Examples include the NXP BLF2425M and BLF25M series of power transistors. Because using RF/microwave energy for heating applications will require high efficiency, these new transistors boast power-added-efficiencies in excess of 52%, with output power levels between 12 and 350 W.

An example of one of these devices, model BLF2425M7LS250P, is a silicon laterally diffused metal oxide semiconductor (LDMOS) power transistor designed for ISM heating at 2400 to 2500 MHz (Fig. 2). Under Class AB operation at 2450 MHz, it can deliver 250 W CW output power with 51% power-added efficiency when powered by a +28-VDC supply. It provides 15-dB typical gain.

2. Model BLF2425M7LS250P is a silicon-LDMOS power transistor designed for ISM heating at 2400 to 2500 MHz.

Representatives from the company noted that this portfolio of “heating transistors” was developed with inputs from key customers. They need RF/microwave-powered solutions for home appliances for cooking, medical electronics systems, and even automotive engines (thus, the RF spark-plug devices). These new transistors, which are based on the firm’s latest silicon LDMOS semiconductor +28-VDC processes, promise to deliver both device durability and reliability.

According to NXP’s Director of Marketing for the RF Power Product Line, “Over the last 10 years, engineers exploring the ways to harness RF energy have had to be content using brute-force magnetrons with extremely limited or next to no control—with absolute power level the only parameter they could adjust for industrial, scientific, and medical applications. With our new dedicated 2.45-GHz ISM portfolio, we’re providing solid-state RF power transistors that have been optimized for this important frequency band with a complete set of options for different power levels, control, and cost. We look forward to working with our customers as they use RF energy as a cleaner, more efficient, and controllable power source for exciting new applications.” Time will tell if RF/microwave heating—especially from solid-state devices—makes sense for this range of applications.